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In the example:

He came to a sudden halt in front of a tall suit of armor. He had been so busy getting away from the library, he hadn't paid attention to where he was going. Perhaps because it was dark, he didn't recognize where he was at all. There was a suit of armor near the kitchens, he knew, but he must be five floors above there.

From the fact he knew that there was a suit of armor near the kitchens,
he thought he must be five floors above the library.
Then, why the two clauses are connected with ‘but’?
What does this but mean?
+1
'However'. He is wondering where this other suit of armor might be, since it is not the one near the kitchen.
Comments  
Thank you very much.